Help me understand Renderers, especially in 2.8
Thank you for stating your question so clearly.
1.) Many different representation engines in Blender 2.8 will allow you to navigate, differentiate, represent volumes (smoke, fire, etc), heavy dense scenes and realistic light bounce materials and environments. You´ll learn in time how to use them to the best of the representation for each case.
2.) Workbench: when you want to see lot of objects inside an archviz scene, EEVEE: if you want to see the completed materials/textures in real world light conditions), etc..
3.) Eevee doesn´t support adaptive subdivision like in cycles at the moment. No further notice have been given about this, don´t know if it will be implemented, as the engine per se, is different than Cycles (EEVEE is screen space based effects only)
4.) Right not there isn´t a specific tutorial, but upvote me as answer and click on the left side checkmark icon of this post because:
1. When you model: Work in workbench using wireframe
2. When you sculpt: Use workbench in solid mode
3. When you create your materials/textures: use look and dev mode (flat light or Hdri or Studio light)
4. When you´re lighting: Use look and dev
5. When your're ready to render switch to EEVEE or CYcles.

There are some nodes that will only work for eevee, not for cycles, and backwards.
The only way to know what to use is to ask around or watch tutorials. No guides specifically filtering those nodes have been created up until this moment.

4.) To answer this you need to search for "screen space effects". Which in addition "What the camera sees is computed into a 2D space, takes the info of any normal or UV space around it, and applies a certain code to "compile" a result of light calculation".

5) There are many tutorials. I´d bid you to search for "eevee light reflection probes" to start with.

6) Experimental only means not every user has a necessity to use them, so it´s best for the system to have it "dormant" until this function is needed. Blender makes a "wake up" call to many functions in it´s code. They are activated at the time of need. This is the reason why you open Blender, loads up necessary components and starts up in less than 5 seconds. Compare that to a larger (fully loading .dll applications that can take up to 1:30 minutes to load up).

7) World machine is superb. Just keep generating procedurals using Cycles. The only way for now. Unless you bake a high poly displacement and THEN take that huge poly count into EEVEE. But this is just a work"around" method. 

BFCT_Schiller